Stacked die package for MEMS resonator system

ABSTRACT

A stacked die package for an electromechanical resonator system includes a chip that contains an electromechanical resonator bonded onto the control chip for the electromechanical resonator by a thermally and/or electrically conductive epoxy. In various embodiments, the electromechanical resonator can be a micro-electromechanical system (MEMS) resonator or a nano-electromechanical system (NEMS) resonator. Packaging configurations that may include the chip that contains the electromechanical resonator and the control chip include chip-on-lead (COL), chip-on-paddle (COP), and chip-on-tape (COT) packages. The stacked die package provides small package footprint and/or low package thickness, as well as low thermal resistance and a robust conductive path between the chip that contains the electromechanical resonator and the control chip.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of the United States provisional patentapplication titled, “Packages and/or Packaging Techniques forMicroelectromechanical Devices,” filed on Jun. 15, 2006 and having Ser.No. 60/813,874. The subject matter of this related application is herebyincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate generally to the fabricationof packaged timing references and particularly to a packagingconfiguration for micro-electromechanical systems (MEMS) andnano-electromechanical systems (NEMS) resonator systems.

2. Description of the Related Art

Quartz resonator systems are used for timing applications in manyelectronic devices, including cell phones, automotive systems, gameconsoles, broadband communications, and almost any other digital productavailable. As quartz resonators decrease in size to meet the sizeconstraints of new applications, the unit cost of quartz resonatorsincreases while their reliability decreases. This is because somemanufacturing processes become increasingly problematic with decreasingsize, such as the formation and testing of a quartz resonator's hermeticseal. In addition, the reduction in size of quartz resonators may noteven be practicable beyond a certain minimum size, given the mechanicalconstraints of the manufacturing processes currently in use.

Micro-electromechanical systems, or MEMS, are also used as resonatorsfor electronic devices. MEMS include devices ranging in size from themicrometer to the millimeter scale. NEMS devices are similar to MEMS,but significantly smaller in size—from the sub-micrometer scale down tothe nanometer scale. MEMS and NEMS are distinguished from comparablysized electronic devices, such as integrated circuits, in that MEMS andNEMS include both electrical and moving mechanical components that aregenerally fabricated together using micro-machining techniques.

One feature of MEMS devices in general, and MEMS resonator systems inparticular, is that as MEMS resonators decrease in size, the unit costof each MEMS resonator decreases, while the reliability of the smallerMEMS device is largely unaffected. This is because more MEMS devices canbe manufactured on a given silicon substrate as the size of the MEMSdevice is reduced, thus defraying the per-substrate manufacturing costover a larger number of MEMS devices. And, as long as manufacturingdesign rules are not exceeded, the performance and reliability ofsmaller MEMS devices is generally as robust as that of larger MEMSdevices. Therefore, due to these cost- and performance-related reasons,there is an on-going effort to develop MEMS packaged timing referencesto replace quartz, ceramic, solid-state, and other types of packagedtiming references in numerous electronic device applications.

Accordingly, there is a need in the art for a chip package for MEMS andNEMS resonator systems that allows for the replacement of conventionalpackaged timing references in existing applications and enables the useof MEMS packaged timing references in applications that are impracticalfor quartz and other types of packaged timing references.

SUMMARY OF THE INVENTION

One embodiment of the present invention sets forth a packaging structurefor an electromechanical resonator system. The packaging structureincludes a control chip for an electromechanical resonator thatcomprises a micro-electromechanical system (MEMS) ornano-electromechanical system (NEMS) resonator, and a second chip thatincludes the electromechanical resonator and is mounted on the controlchip in a stacked die configuration, wherein the second chip isthermally coupled to the control chip by a thermally conductive epoxy.

One advantage of the disclosed packaging structure is that it provides asmall package footprint and/or small package thickness as well as lowthermal resistance and a robust electrically conductive path between thesecond chip and the control chip. The disclosed package may therefore beused in lieu of alternate packaged timing references in variouselectronic devices due to cost, reliability, and size constraints.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1A illustrates a schematic cross-sectional view of a stacked dieCOL package configuration, according to an embodiment of the invention.

FIG. 1B illustrates a schematic cross-sectional view of a stacked dieCOL package configuration with a downset chip, according to anotherembodiment of the invention.

FIG. 1C illustrates a flow chart outlining a process sequence forproducing the COL package as illustrated in FIG. 1A.

FIG. 2A illustrates a schematic cross sectional view of a stacked dieCOP package configuration, according to an embodiment of the invention.

FIG. 2B illustrates a flow chart outlining a process sequence forproducing the COP package as illustrated in FIG. 2A.

FIG. 3A illustrates a schematic cross sectional view of a stacked dieCOT package configuration, according to an embodiment of the invention.

FIG. 3B illustrates a flow chart outlining a process sequence forproducing the COT package as illustrated in FIG. 3A.

For clarity, identical reference numbers have been used, whereapplicable, to designate identical elements that are common betweenfigures. It is contemplated that features of one embodiment may beincorporated in other embodiments without further recitation.

DETAILED DESCRIPTION

Embodiments of the invention contemplate stacked die packageconfigurations for a MEMS resonator and its associated control chip thatprovide small package footprint and/or low package thickness. Thesestacked die package configurations further provide low thermalresistance and a robust electrically conductive path between theresonator chip and the control chip. Stacked die configurations includechip-on-lead (COL), chip-on-paddle (COP), and chip-on-tape (COT)packages. MEMS resonators contained in COL, COP, or COT stacked diepackages, according to embodiments of the invention, may be beneficiallyused in lieu of quartz, ceramic, solid-state and other types of packagedtiming references, due to the cost, reliability, and size constraints ofthese packaged timing references. In addition, the stacked die packagesprovided herein enable “drop-in” replacement of quartz packaged timingreferences used in existing applications, i.e., the form-factor and leadconfiguration of a packaged MEMS resonator can be made essentiallyidentical to quartz-based packaged timing references. Thus, thereplacement of a quartz packaged timing reference in an electronicdevice with a functionally equivalent MEMS packaged timing reference istransparent to the architecture of the device, and therefore nomodifications to the device are necessary to accommodate the MEMSresonator package.

Chip-on-Lead Stacked Die

FIG. 1A illustrates a schematic cross-sectional view of a stacked dieCOL package configuration, according to an embodiment of the invention.COL package 100 includes a MEMS chip 101, a control chip 102, and aplurality of leads 103, which are assembled and enclosed inside a moldcompound 104. MEMS chip 101 includes a MEMS device layer 101A and a bulklayer 101B and is mounted onto control chip 102 with a conductive epoxy105, as shown. A fully formed MEMS resonator (not shown) is contained inMEMS device layer 101A and is electrically coupled to control chip 102by a plurality of bonding wires 106, thereby allowing control chip 102to power, control, and sense the output of the MEMS resonator. In theexample illustrated in FIG. 1A, control chip 102 is a CMOS chip, butother micro-electronic control chips are also contemplated. Control chip102 is mounted onto the leads 103. An electrically non-conductive epoxy107 bonds control chip 102 to leads 103, and electrically insulatescontrol chip 102 from leads 103. A plurality of bonding wires 108electrically couples control chip 102 to the appropriate leads 103 forthe proper operation of control chip 102, e.g., power, ground, resonatoroutput signal, etc. Each lead 103 has an electrical contact surface 109exposed on the bottom of COL package 100 to facilitate connection to aboard (not shown) contained in a parent electronic device.

Because the performance of MEMS resonators is temperature sensitive,control chip 102 contains a temperature sensor to compensate fortemperature changes experienced by the MEMS resonator contained in theMEMS device layer 101A. Proper operation of the MEMS resonator thereforedepends on a short thermal path between the temperature sensor incontrol chip 102 and the MEMS resonator itself. Conductive epoxy 105serves to mechanically bond MEMS chip 101 onto control chip 102, whilethermally coupling MEMS chip 101 to control chip 102. In addition,conductive epoxy 105 may electrically couple MEMS chip 101 with controlchip 102 via apertures 110 formed through passivation layer 102A ofcontrol chip 102. Passivation layer 102A is an electrically insulatinglayer formed as a top layer of control chip 102 to protect themicro-electronic devices contained therein. Before MEMS chip 101 isbonded onto control chip 102, apertures 110 are formed in passivationlayer 102A by lithographic methods known in the art. Conductive epoxy105 then forms one or more conductive paths between the MEMS chip 101and control chip 102, as shown. These conductive paths prevent anypotential difference from developing between MEMS chip 101 and controlchip 102. As used herein, “conductive” is defined as being sufficientlydissipative of electric charge to act as a conductive path for a staticelectric charge, i.e., having a resistivity of no more than about 1 to10 Megohm-cm.

Maximizing the surface area of MEMS chip 101 and control chip 102 thatare in contact with conductive epoxy 105 enhances the thermal andelectrical coupling provided by conductive epoxy 105. In the exampleshown in FIG. 1A, the entire backside of MEMS chip 101 and most of thesurface of control chip 102 are in contact with conductive epoxy 105. Inaddition, the thermal and electrical conductivity of conductive epoxy105 may be enhanced by the presence of conductive particles, such assilver particles, included therein. Such thermally conductive epoxiesare known in the art for application to the backside of COP packages forCMOS and other chips, but are typically not used as stacking epoxies dueto their inherent rigidity and/or abrasiveness. To address this concern,conductive epoxy 105 is selected to have a coefficient of thermalexpansion that is relatively close to that of silicon (Si), to minimizethe mechanical stress induced by changes in temperature of the MEMSresonator and control chip 102, which in turn reduces the force impartedon passivation layer 102B. In this way, damage to passivation layer 102Band control chip 102 is much less likely to occur when COL package 100undergoes significant temperature changes. In one embodiment, conductiveepoxy 105 has a coefficient of thermal expansion between about 2×10⁻⁶/°C. and about 170×10⁻⁶/° C. Examples of electrically and thermallyconductive epoxies that may be used as conductive epoxy 105 includeHysol® QMI 505MT and Hysol® QMI 519.

In addition to COL package 100, other stacked die COL packages arecontemplated for forming a compact and robust MEMS resonator package.For example, the MEMS chip 101 may be mounted to leads 103 and controlchip 102 may then be mounted onto MEMS chip 101. In another example,MEMS chip 101 and control chip 102 may only be partially stacked, orpositioned in an asymmetrical configuration.

FIG. 1B illustrates another stacked die COL configuration contemplatedby embodiments of the invention. COL package 190 is mounted to leads 193in a downset chip configuration, as shown, and generally shares a numberof substantially similar elements with COL package 100, illustrated inFIG. 1A. Identical reference numbers have been used, where applicable,to designate the common elements between COL package 100 and COL package190. Advantages of COL package 190 include a lower cross-sectionalprofile and a broader process window for wirebonding than can beprovided by a standard COL package. Leads 193 are fabricated with aninset cavity 194, and MEMS chip 101 and control chip 102 are positionedinside inset cavity 194 when mounted onto leads 193. In this way, thecross-sectional profile, or thickness, P, of COL package 190 issubstantially reduced compared to COL package 100. In addition, thewirebonding process is more easily and reliably performed on COL package190 than COL package 100 for two reasons. First, an upper surface 195 ofcontrol chip 102 can be substantially aligned with upper surface 196 ofleads 193, which may decrease the time necessary to complete thewirebonding process. Second, leads 193 generally form a more rigidsupport structure for control chip 102 during the wirebonding processthan the more cantilevered configuration of leads 103 in COL package100, thereby increasing the process window of the wirebonding process.An alternate COL package option includes unetched leads, whereby thechips are neither cantilevered nor downset.

FIG. 1C illustrates a flow chart outlining a process sequence 120 forproducing COL package 100 as illustrated in FIG. 1A. Process steps121-123 may be carried out in parallel, as shown.

In step 121, a MEMS device die substantially similar to MEMS chip 101 inFIG. 1A is prepared for packaging. First, a MEMS device die containing aMEMS resonator is fabricated on a substrate using deposition, etching,and lithographic methods commonly known in the art. A plurality of diesmay be fabricated on the substrate simultaneously. Next, a thinningprocess, such as a backgrind process, is performed on the substrate,followed by an optional polishing process. Lastly, the MEMS device dieis diced from the substrate using a process similar to that forsingulating integrated circuit (IC) chips from a silicon wafer.

In step 122, a leadframe containing leads substantially similar to leads103 in FIG. 1A is fabricated. The leadframe is formed from a platedmetallic substrate, such as copper plated with NiPdAu, using etching andlithographic methods commonly known in the art. Similar to thefabrication of a MEMS device die described in step 121, the leads for aplurality of COL packages may be fabricated from a single substrate atonce.

In step 123, a control die similar to control chip 102 is prepared forpackaging. The control die, which is a conventional integrated circuitdie, is fabricated and prepared via a process similar to step 121, i.e.,deposition, etching, lithography, thinning, and dicing are used toproduce one or more singulated control dies from a silicon substrate. Inaddition, the control die is further prepared for packaging by thescreen printing of an electrically non-conductive epoxy on the back ofthe silicon substrate prior to dicing. Alternatively, the electricallynon-conductive epoxy may instead be deposited onto the leadframedirectly as part of fabricating the leadframe in step 122.

In step 124, the control die is attached to the leadframe with theelectrically non-conductive epoxy. As noted above, the electricallynon-conductive epoxy may be screen printed to the backside of thecontrol die in step 123 or applied to the leadframe in step 122.

In step 125, a conductive epoxy, which is substantially similar toconductive epoxy 105 in FIG. 1A, is deposited in preparation forattaching the MEMS die onto the control die in a stacked dieconfiguration. The conductive epoxy may be deposited onto the backsideof the MEMS die or onto the requisite surfaces of the control die.

In step 126, the MEMS die is attached to the control die in a stackeddie configuration using methods commonly known in the art.

In step 127, the MEMS die, the control die, and the leadframe arewirebonded as required to electrically couple the two dies to each otherand to the leadframe. Because wirebonding the MEMS die and the controldie involves pressing a ball bond or other wire onto a substantiallycantilevered substrate, i.e., the leadframe, the process window for thewirebonding process may be substantially reduced compared toconventional wirebonding processes. For example, the force required toproduce good electrical contact may be relatively close to the forcerequired to plastically deform, and therefore damage, portions of theleadframe or control die. Alternatively, a leadframe having a downsetchip configuration may be used to address this issue.

In step 128, the stacked die package is enclosed in a protective moldcompound substantially similar to mold compound 104 in FIG. 1A.

In step 129, the stacked die package is singulated out of the leadframesubstrate using methods commonly known in the art.

Other sequences in addition to process sequence 120 are contemplated forproducing COL package 100. For example, the MEMS die prepared in step121 may be attached and wirebonded to the control die before the controldie is attached to the leadframe in step 124. In another example, partof step 121, i.e., MEMS die preparation, may include the deposition ofconductive epoxy onto the backside of the MEMS substrate prior to dicingthereof. In this case, deposition of the epoxy may include screenprinting or other methods known in the art.

The stacked die COL structure of COL package 100 is a compact, robustpackaging structure for a MEMS resonator and control chip, made possibleby the electrical and thermal conductive paths between MEMS chip 101 andcontrol chip 102 that are formed by conductive epoxy 105. Hence, the useof an electrically and/or thermally conductive epoxy having acoefficient of thermal expansion substantially the same as siliconenables the packaging of a MEMS chip and a control chip as a COL stackeddie structure. With a stacked die structure, COL package 100 can beconfigured with a footprint that is quite small relative to the size ofMEMS chip 101 and control chip 102. Because of its inherently smallfootprint, COL package 100 may be used as a drop-in replacement forapplications utilizing small quartz resonator packages, such as 2.5 mm×2mm QFN packages, among others. In addition, the stacked die structure ofCOL package 100 also allows the packaging of MEMS resonators withpackages that have significantly smaller footprints than packaged timingreferences known in the art and smaller footprints than MEMS resonatorspackaged in standard chip packages. These smaller packages enable theuse of a MEMS resonator packaged timing reference in developingapplications requiring a thickness of less than 350 μm and/or afootprint of less than 1.6 mm×2.0 mm, which are impracticable for othertypes of packaged timing references, such as solid-state, ceramic, orquartz packaged timing references.

The ability to reduce the size of a MEMS resonator package is beneficialfor other reasons as well. Smaller packages are inherently morereliable, since they have less surface area for moisture ingress tocontaminate epoxies and metal joints. In addition, smaller packages aresubject to less thermally induced stress between the package and theboard onto which the package is mounted or soldered. This is because thethermally induced stress produced between joined objects consisting ofdissimilar materials is proportional to size of the objects. Further,smaller packages are more rigid, i.e., a given quantity of stress causesless strain and deflection of internal components in a smaller packagethan on those in a larger package. Hence, a smaller package undergoesless thermally induced stress and is also less sensitive to such stress.Because MEMS devices are very sensitive to strain and deflection, theirreliability and accuracy is substantially improved when the package sizeis minimized.

Chip-on-Paddle Stacked Die

FIG. 2A illustrates a schematic cross sectional view of a stacked dieCOP package configuration, according to an embodiment of the invention.COP package 200 shares a number of substantially similar elements withCOL package 100 illustrated in FIG. 1A. Identical reference numbers havebeen used, where applicable, to designate the common elements betweenCOL package 100 and COP package 200.

As shown in FIG. 2A, MEMS chip 101 is mounted on control chip 102 withconductive epoxy 105, and both chips are wirebonded to each other and toa plurality of leads. As described above in conjunction with FIG. 1A,conductive epoxy 105 electrically couples MEMS chip 101 to control chip102 via apertures 110, mechanically bonds the chips, and thermallycouples the chips. In contrast to leads 103 of COL package 100, leads203 do not structurally support control chip 102 and MEMS chip 101.Instead, control chip 102 is mounted on and supported by a die paddle230, which is electrically and physically isolated from one or more ofthe leads 203 as shown.

Die paddle 230 serves as the primary region of thermal input and outputfor COP package 200. Because of this, a thermally conductive andelectrically conductive epoxy 207 may be used to bond control chip 102to die paddle 230. Alternatively, epoxy 207 may also be electricallyinsulative for some applications. Die paddle 230 extends beyond theedges of control chip 102, as shown, producing an overlap region 231.Overlap region 231 is a necessary feature of COP package 200 due todesign rules known in the art regarding the structure of COP packagesfor IC or other chips. Also, because leads 203 and die paddle 230 areformed from what is initially a single continuous metallic substrate,one or more of leads 203 are separated from die paddle 230 by a minimumgap 232, according to standard design rules known in the art for theleadframe etch process. Etch design rules, such as the maximum aspectratios of etched features, are necessary for the reliable separation ofleads 203 from the die paddle 230 during the etch process. When suchdesign rules are violated, minimum gap 232 may be incompletely formed,and die paddle 230 may not be electrically isolated as necessary fromone or more of leads 203, thereby rendering the MEMS resonator in MEMSchip 101 inoperable. It is noted that, for clarity, overlap region 231and minimum gap 232 have not been drawn to scale in FIG. 2A and aregenerally much larger relative to control chip 201 than shown.

It is known in the art that, for a given chip footprint, COP packagesare inherently larger than COL packages. This is due to overlap region231 and minimum gap 232, which make up a significant portion of COPpackage footprint, and therefore largely dictate the minimum size of aCOP package, regardless of the sizes of the MEMS chip 101 and thecontrol chip 102. However, embodiments of the invention contemplate astacked die COP package for MEMS resonators to better facilitate thedrop-in replacement of existing quartz resonator applications. Packagedquartz resonators for existing applications may be relatively large,e.g., 5 mm×7 mm, and therefore do not require the smaller footprintbenefit of a COL package, as described above in conjunction with FIG.1A.

FIG. 2B illustrates a flow chart outlining a process sequence 220 forproducing COP package 200 as illustrated in FIG. 2A. A number of theprocess steps for process sequence 220 are substantially similar to thecorresponding process steps in process sequence 120, described above,and are therefore provided with identical reference numbers, whereapplicable.

In step 121, a MEMS device die substantially similar to MEMS chip 101 inFIG. 1A is prepared for packaging. This process step is described abovein conjunction with FIG. 1C.

In step 222, a leadframe substantially similar to the leadframecontaining leads 203 in FIG. 2A is fabricated. With the exception of theparticular features formed into the metallic substrate, this processstep is substantially identical to step 122, described above inconjunction with FIG. 1C. Because the features formed into a leadframesubstrate for a COP package, i.e., the die paddle and leads, are easierto fabricate than the more complicated features of a COL packageleadframe, conventional etching and lithographic methods commonly knownin the art may be used for step 222.

In step 223, a control die similar to control chip 102 is prepared forpackaging. This process step is substantially similar to step 123,described above in conjunction with FIG. 1C, except that theelectrically non-conductive epoxy may also be selected to beelectrically and/or thermally conductive. In this way, control chip 102is thermally coupled to die paddle 230, thereby allowing die paddle 230to act as the primary region of thermal input and output for COP package200. Electrically conductive epoxy allows the control chip 102 to beelectrically coupled to the die paddle 230.

In step 124, the control die is attached to the leadframe with thethermally conductive, electrically conductive epoxy. This process stepis described above in conjunction with FIG. 1C. Alternately, theconductive epoxy could be non-electrically conductive.

In step 125, a conductive epoxy, is deposited in preparation forattaching the MEMS die onto the control die in a stacked dieconfiguration. This process step is also described above in conjunctionwith FIG. 1C.

In step 126, the MEMS die is attached to the control die in a stackeddie configuration using methods commonly known in the art. This processstep is also described above in conjunction with FIG. 1C.

In step 127, the MEMS die, the control die, and the leadframe arewirebonded as required to electrically couple the two dies to each otherand to the leadframe. The wirebonding process for COP packaging iscommonly known in the art, and is further described above in conjunctionwith FIG. 1C.

In step 128, the stacked die package is enclosed in a protective moldcompound substantially similar to mold compound 104 in FIG. 1A. Thisprocess step is described above in conjunction with FIG. 1C.

In step 129, the stacked die package is singulated out of the leadframesubstrate using methods commonly known in the art. This process step isalso described above in conjunction with FIG. 1C.

Other sequences in addition to process sequence 220 are contemplated forproducing COP package 200. For example, the MEMS die prepared in step121 may be attached to the control die before the control die isattached to the leadframe in step 124. In another example, part of step121, i.e., MEMS die preparation, may include the deposition ofconductive epoxy onto the backside of the MEMS substrate prior to dicingthereof.

Chip-on-Tape Stacked Die

FIG. 3A illustrates a schematic cross sectional view of a stacked dieCOT package configuration, according to an embodiment of the invention.COT package 300 shares a number of substantially similar elements withCOL package 100 illustrated in FIG. 1A. Therefore, identical referencenumbers have again been used, where applicable, to designate the commonelements between COL package 100 and COT package 300.

As shown in FIG. 3A, MEMS chip 101 is mounted on control chip 102 withconductive epoxy 105, and both chips are wirebonded to each other and toleads 303. As described above in conjunction with FIG. 1A, conductiveepoxy 105 electrically couples MEMS chip 101 to control chip 102 viaapertures 110, mechanically bonds the chips, and thermally couples thechips. In contrast to COL package 100 and COP package 200, control chip102 and leads 303 are mounted onto an adhesive tape 330, therebyenabling a lower cross-sectional profile, P, for COT package 300 than ispracticable for COL and COP MEMS resonator packages. In this way, thecross-sectional profile P of COT package may be 350 μm or less. Thecontrol chip 102 may be bonded directly to the adhesive tape 330, or anepoxy layer may be deposited between the control chip 102 and theadhesive tape 330. Positioning leads 303 and control chip 102 as shownon adhesive tape 330 electrically and physically isolates leads 303 fromcontrol chip 102. Control chip 102 and MEMS chip 101 are wirebonded toeach other and to leads 303 as shown. In some applications, adhesivetape 330 is removed after mold compound 104 is formed around MEMS chip101 and control chip 102, thereby exposing an exposed chip surface 331of control chip 102 and electrical contact surface 109 of leads 303. Inother applications, tape 330 is left in place and electrical contact ismade to electrical contact surface 109 via metallic layers deposited onadhesive tape 330.

FIG. 3B illustrates a flow chart outlining a process sequence 320 forproducing COT package 300 as illustrated in FIG. 3A. A number of theprocess steps for process sequence 320 are substantially similar to thecorresponding process steps in process sequence 120, described above,and are therefore provided with identical reference numbers, whereapplicable.

In step 121, a MEMS device die substantially similar to MEMS chip 101 inFIG. 1A is prepared for packaging. This process step is described abovein conjunction with FIG. 1C.

In step 322, a leadframe substantially similar to the leadframecontaining leads 303 in FIG. 3A is fabricated. With the exception of theparticular features formed into the metallic substrate, this processstep is substantially identical to step 122, described above inconjunction with FIG. 1C.

In step 323, a control die similar to control chip 102 is prepared forpackaging. This process step is substantially similar to step 123,described above in conjunction with FIG. 1C, except that the epoxyapplied to the backside of the silicon substrate may be eitherelectrically conductive or electrically non-conductive, depending on theapplication for COT package 300.

In step 324, the control die for the COT package are attached to anadhesive tape substantially similar to adhesive tape 330 in FIG. 3A.

In step 125, a conductive epoxy is deposited in preparation forattaching the MEMS die onto the control die in a stacked dieconfiguration. This process step is described above in conjunction withFIG. 1C.

In step 126, the MEMS die is attached to the control die in a stackeddie configuration using methods commonly known in the art. This processstep is also described above in conjunction with FIG. 1C.

In step 327, the MEMS die, the control die, and the leads are wirebondedas required to electrically couple the two dies to each other and to theleads mounted on the adhesive tape using wirebonding processes for COTpackaging commonly known in the art.

In step 128, the stacked die package is enclosed in a protective moldcompound substantially similar to mold compound 104 in FIG. 1A. Thisprocess step is described above in conjunction with FIG. 1C.

In step 329, the stacked die package is singulated out of the leadframeusing methods commonly known in the art.

Other sequences in addition to process sequence 320 are contemplated forproducing COT package 300. For example, a MEMS chip may first be mountedonto a control chip as described in step 126, then the control chip maybe mounted onto the adhesive tape as described in step 324. In addition,the MEMS chip may be wirebonded to the control chip before the controlchip is mounted onto the adhesive tape.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A packaging structure for an electromechanical resonator system, thepackaging structure comprising: a control chip for amicroelectromechanical resonator; an electromechanical resonator chipthat (i) includes the microelectromechanical resonator and (ii) ismounted on the control chip in a stacked die configuration, wherein theelectromechanical resonator chip is physically coupled to the controlchip by an electrically conductive epoxy; a lead frame including atleast one electrical lead having a mounting surface and an electricalcontact surface, wherein the stacked die configuration is (i) mounted onthe mounting surface of the electrical lead and (ii) physically coupledthereto by an electrically insulating epoxy, and wherein the stacked dieconfiguration is electrically coupled to the electrical lead; and a moldcompound, enclosing at least a portion of the lead frame and the stackeddie configuration of the control chip and the electromechanicalresonator chip, wherein (i) the mounting surface of the electrical leadand the portion of the stacked die configuration which is mountedthereon are enclosed in the mold compound and (ii) the electricalcontact surface of the electrical lead is an externally exposed portionof the packaging structure.
 2. The packaging structure of claim 1,wherein the control chip includes a passivation layer disposed thereonand wherein a portion of a peripheral surface of the control chip notincluding the passivation layer contains at least a portion of theelectrically conductive epoxy which electrically couples the controlchip to the electromechanical resonator chip.
 3. The packaging structureof claim 1, wherein the electrically conductive epoxy has an electricalresistivity of less than about ten Megaohms.
 4. The packaging structureof claim 1, wherein the electrically conductive epoxy electricallycouples portions of the control chip to the electromechanical resonatorchip.
 5. The packaging structure of claim 4, wherein the bulk layer ofthe electromechanical resonator chip is bonded to a passivation layer ofthe control chip by the electrically conductive epoxy.
 6. The packagingstructure of claim 5, wherein the electrically conductive epoxyelectrically couples the electromechanical resonator chip to the controlchip via apertures in the passivation layer of the control chip.
 7. Thepackaging structure of claim 1, wherein the at least one electrical leadis wirebonded to the control chip.
 8. The packaging structure of claim7, wherein the control chip has a first surface coupled to theelectromechanical resonator chip and a second surface that comprises aportion of an external surface of the packaging structure.
 9. Thepackaging structure of claim 1, wherein the lead frame includes aplurality of electrical leads, wherein each conductive lead has anexposed electrical contact surface positioned on the external surface ofthe packaging structure.
 10. The packaging structure of claim 1, whereinthe packaging structure has a cross-sectional profile having a height ofless than 350 μm.
 11. The packaging structure of claim 1, wherein thepackaging structure has a footprint that is equal to or less than 3.2mm².
 12. The packaging structure of claim 11, wherein the footprint hasa length that is equal to or less than 2.0 mm and a width that is equalto or less than 1.6 mm.
 13. The packaging structure of claim 1, furtherincluding a first electrically conductive bonding wire, disposed withinthe mold compound, to electrically couple circuitry disposed on or inthe control chip to the electromechanical resonator of theelectromechanical resonator chip.
 14. The packaging structure of claim13, further including: a second electrically conductive bonding wiredisposed within the mold compound, and wherein the second electricallyconductive bonding wire electrically couples the control chip and theportion of the electrical lead that is disposed in the mold compound.15. The packaging structure of claim 1, wherein the control chipincludes a temperature sensor.
 16. The packaging structure of claim 15,wherein the control chip compensates for changes in temperature of themicroelectromechanical resonator using data from the temperature sensor.